Turbomachinery rotors are often designed to operate at rotational speeds at, near, or above at least one of the resonant frequencies of system. Due to residual unbalance, the rotors often exhibit a synchronous response to rotation that tends to increase in amplitude at frequencies nearing the system natural frequencies, especially when the resonances are lightly damped. Further, even when the rotor is operating sufficiently far away from its resonant frequencies, periodic excitation forces may be incident on the rotor, which may have a frequency at or near the resonant frequency of the rotor. The application of such excitation forces may lead to instability in the system, such that the vibration grows and can cause damage to the machinery.
Damper bearings are employed in turbomachinery to provide damping and stiffness to the rotor, thereby supporting the rotor and reducing vibration. Such damper bearings are typically mounted on squeeze film or metal-mesh (i.e., mechanical spring) systems to transmit bearing loads to the bearing supports, while increasing total system damping. One drawback to these traditional damper bearings, however, is that they must be supported by a relatively flexible spring (or equivalent structure). Accordingly, the static load applied by the rotor on this spring introduces rotor eccentricity with respect to the bearing and any other static components. As such, system designers are often required to add additional components to reduce such eccentricity, which adds complexity and additional space requirements to the system.
Active magnetic damper bearings have been proposed to overcome these difficulties by applying variable force to the rotor to control its position in real time. These bearings, however, require a complex system of electromagnets, sensors, sensor wiring, power wiring, power amplifiers, a controller, and a back-up power supply, etc. Even with redundancy components, however, the active nature and complexity of the system increases the likelihood of failure and increases system cost.
What is needed is a damper that does not act as a static rotor-support or centering device but that provides effective damping, without suffering from a significantly increased likelihood of failure and/or cost.